Tone control



Jan. 15,1946.

E. B; P sSow TONE CONTROL Filed Oct. 14, 1944 INVEINTOR EDWARD B.PAYSSOW BNM HIS ATTORNEY Patent'ed Jan. 15, 1946 'rorm ooN'raoL EdwardB. Passow, Park mm, m, assignor to Zenith Radio Corporation,

linois a corporation of Il- Application October 14, 1944, Serial No.558,723

QCLaims.

This invention relates to tone control arrangemcntsof the typeparticularly useful in hearing aid or other portable hearing apparatus.

In hearing aid or other small portable hearing apparatus it isadvantageous to have incorporated in such apparatus a single controlelement readily movable by the wearer to one of a plurality of operatingpositions corresponding to the particular character of tone desired fromsuch apparatus. This advantage in a tone control arrangement should beobtained by using as few electrical and mechanical elements as possible;otherwise, the cost of the component parts is expensive, the apparatusis needlesly heavy and bulky and it may not be so readily carried orconcealed on the person of the user.

It is, therefore, an object of the present invention to provide animproved tone. control arrangement which is inexpensive to'manufacture,readily operable by the user and which does not add materially to theweight or bulkiness of the apparatus with which it co-acts to producedifl'erent characters of tone. f

A further object of the present invention is to provide an improvedinexpensive switch construction having a minimum number of parts andal-' tliming; mass production in a relatively short Still another objectof the present invention is to provide a simple improved tone controlarrangement especially useful in small portable hearing apparatus.

The features of the present invention which are believed to be novel aresetforthwith particularlty in the appended claims. The present inventionitself, both as to'its organization and manner of operation, togetherwith further objects and advantages thereof may best be understood byreierence to the following description taken in connection with theaccompanying drawing in which: I

In Fig. 1 there is illustratedan improved hearing aid circuit,incorporating a high gain amplifler including electron discharge deviceI and a second amplifier including electron discharge device 2. The twoamplifiers successively amplify signals from microphone 3 and impresssuch amplified signals on sound reproducing device 4 in linearlyamplified form. A switch 5, described in detail hereinafter, foreffecting tone control is interconnected with device I.

Sound waves impinging on microphone 3 are transformed into electricalvariations in the main control electrode circuit of discharge device i.The microphone 3 is preferably of the piezo-electric typeas illustrated.The device I greatly amplifies electrical variations producedbymicrophone 3 and the amplified electrical variations are furtheramplified by electron discharge device 2 before being applied to thesound reprogo ducing device 4.

Fig. 1 shows an improved hearing aid circuit incorporating an improvedtone control arrangement in accordance with the present invention;

l'flgs. 2 through 4 show certain elements of the switch shown in Fig. 5in different operating positions thereof; M

Fig. 5 shows a sectional view taken substantially on line ll1of Fig. 1;I

Fig. 6 shows, in enlarged form, the other side of the switchconstruction shown'in Pig. 1;

Figs. 7 and a show respectively plan and eleva tional views of a portionof a portable case having incorporated therein a tone control inaccord'ance with the present invention. 1

Microphone 3, which produces electrical variations in response to soundwaves impinging thereon, is connected between the main control grid iand the grounded filamentary cathode I of discharge device I; and a gridleak resistance 8 is connected in ,parallel circuit relationship withmicrophone I to bypass continuous current flowing around microphone 3between control grid I and the cathode I. Substantially no grid currentflows through resistance I, because the grid 0 is at a negativepotential with respect to the cathode 1, such negative bias potentialbeing provided by connecting grid 8 through resistance 8 to the groundednegative terminal of voltage source 9 across which are connected theopposite terminals of the filamentary cathode I. It is to be understood,of course, that other conven' tional means may be used to establish asuitable continuous operating potential between grid 8 and cathode I andsuch means may be a battery or a voltage drop produced by space currentflowing through discharge device I. Resistance l0,

-. much smaller than resistance I, is also connected in parallel tomicrophone I through switch 5 for a different purpose than resistance Ias is described hereinafter.

Electron discharge device I is of the pentagrid type in commercial useand may, for example, be

. of the type commonly knownas the 1R5; The

185, but within the scope of the present inven-- tiolr any dischargedevice which performs equivalent functions is suitable. Large gainisrealized when device I is connected in the manner hereinafter describedand such large gain is substantially independent of signals appliedbetween the main control grid 6 and cathode of discharge device I.

In general, device I is connected so as to be effectively two amplifiersin cascade with regeneration between the two amplifiers. In addition tothe main control grid 6, discharge device It has what is termed a secondcontrol grid H, a suppressor grid l2 connected to the cathode l, a mainanode l3, and a pair of screen electrodes I4 and i5 on opposite sides ofthe second control grid I Operating continuous potentials for device 1are supplied from a voltage source It whose negative terminal isgrounded and whose positive ter minal is connected to the main anode I:of discharge device i through a series circuit including adjustablevoltage dropping resistance l7 and output coupling resistance ii. Theelectrodes it and 15 are cozmected together and are maintained positivewith respect to the cathode by connection. to the positive terminal ofvoltage source l6 through the series circuit including voltage droppingresistance I! and coupling re sistance is. The continuous operatingpotential of the second control grid 1 i is stabilized connecting it toground and the cathode I through resistance 20, which also serves as acoupling re sistance for audio frequency current as descrii: iihereinafter.

When alternating current signals are impressed between the main controlgrid 8 and cathode 1 substantially all of the, alternating outputvoltage appears across output coupling resistance l8, a bypasscapacitance II of low reactance being connected between the groundedcathode of discharge device I and the lower terminal of resistance isremoved from the main anode l3. Potential variations'on electrodes H andi due to an alternating voltage applied between main control grid 8 andcathode I are impressed on the second control grid H through a couplingcapacitance 22 connected between grids H and I! and grid ll.

Therefore, alternating voltages applied directly to control grid 6 andindirectly to control grid ll cause alternating output signals to appearacross resistance 18, which output signals are then up plied to the gridcircuit of another linearly am pliiying discharge device 2. Couplingcapacitance 23 and input resistance 24 are connected in series and theseries circuit formed thereby is connected in parallel circuitrelationship to the series circuit formed by output coupling resistancel8 and low reactanoe bypass capacitance 2|. Capacitance 23 is ofrelatively low reactance and serves essentially as a means for blockingthe flow or continuous current from source Is to resistance 24. Thealternating voltage developed across resistance u through condenser 13is applied between the first or control grid 28 and cathode 28 ofdischarge device 2 so as to control the space current therein, whichcurrent norrr iii fact that voltage sour is preferably of the n. Jhaving its supncssor grid 00 connects to the cathode it; and with thevoltage source ii connected between its screen grid ll and cathode 23through a voltage dropping resistance a. The

screen grid II is maintained at constant potential in the presence ofsignals 01 irequency corresponding to audio frequency by means 01' lowreactance bypass capacitance at which is connected between the screengrid II and cathode 28.

Alternating voltages developed across input resistance 24 are amplifiedlinearly by discharge device 2, and appear across the secondary winding34 of transformer 29 which is connected to impress those amplifiedvoltages on the sound reproducing device 4 which is connected across theterminals of secondary winding 34.

The filamentary cathodes of discharge device I and 2 are preferablyheated by current flowin therethrough, as shown in Fig. l, in such casethe cathodes and 26' are connected in parallel circuit relationship tovoltage source 9.

The circuit thus far described is especially useful as a hearing aidcircuit and, because of the high gain obtained by the use of dischargedevice i, only two discharge devices requiring small space current arenecessary for good performance, even though a tone control arrangementin accordance with the present invention is provided. Because of thehigh gain obtained from the use of discharge device I, a highlyefficient and useful hearing aid circuit is provided which requires asmall current drain from voltage source W 51. This is particularly truewhen, as in this instance, resistances H, I! and is are relativelylarge.

The volume of signals reproduced by speaker I may be controlled byadjusting volume control resistance H. An adjustment oi resistance llcauses no substantial non-linearity between the intensity of inputsignals applied to grid 5 and the intensity of signals developed acrossresist: ance (8 over a large range of input signals.

The tone of signals reproduced on sound reproducing device 4 may becontrolled by connecting resistance [0 and, capacitance in the hearingaid circuit thus far described. In general, resistance I0 which may beconnected in parallel circuit relationship to piezoelectric microphone 3serves to reduce the intensity of low frequency signal components, andcapacitance I! which may be connected between the electrode II andcathode 1 of discharge device I serves to reduce the intensity of highfrequency signal components. For this purpose, switch 5 serves toconnect the lead ID of resistance III to ground and alsoserves toconnect the lead 31 o! condenser 35 to ground. Different types of tonemay be produced corresponding to the four positions of the tone controlswitch member 88 shown in Figs. 1-4. That is, switch member or metallicshort circuiting bar 38 having an inwardly disposed body portion 89 andan outwardly extending projecting body 0 is of such shape that in itsclockwise movement, bar 8| assumes positions whereby: (1) as in Fig. lthe capacitance ll alone is connected in the hearing aid circuit andhigh frequencies only are suppressed, and (2) as in Fig. 2 when pro U a.time is connected in the hearing aid circuit only 1 lead ll groundedcauses noattenuation in ordinary sense of the word, but its function isto.

the

present a dlil'erent degree of mismatch as a function of frequency tothe crystal microphone I whose equivalent circuit may be represented bya section including the cathode I, main control grid 6 and electrode it,which electrode operates as the anode of a conventional triode. Thesecond amplifying stage operates as a pentode and includes controlelectrode ll, screen electrode i4, suppressor electrode l2 and anode II.

The overall gain of discharge device I is preferably'adjusted byadjusting the amount of regeneration in discharge device I. It isdesirable to make the amount of regeneration such that linear operationof the stage including device I is assured consistent with as high gainas possible. For purposes of analysis, the two stages may be consideredto be equivalent to two separate discharge devices, a triode and apentode, connected in cascade, wherein the regeneration may be viewed asa negative resistance of the dynatron type connected across the anodeload of the triode.

The regeneration which comes into being is believed to be due tonegative transconductance from second control grid H to the screenelectrodes I4 and I8.

This negative transconductance efiect is produced in accordance with thefollowing constructions: the total cathode space current of device 1 issubstantially independent of the voltage applied to the second controlgrid H, which controls the distribution of current between electrode Iton one side and electrode it and anode II on the other side. Since thetwo electrodes II and it are connected together, substantially equal andopposite effects are produced on the electrode II and on electrode l4and anode i3 by a change of the voltage on the second control grid ii.That is, if electrode ii is made more negative, more space current tendsto ilow to 4 electrode I I and less to electrode I4 and anode I 3. Butthe decrease in current to elect'rode H, which shares its current withanode I3, is less than the increase in current to electrode I5.Therefore, the total current to electrodes l4 and It increases with amore negative potential on electrode .II and current to anode l3decreases.

. Accordingly, the transconductance between sec- 0nd control grid H andanode I3 is positive, and

the transconductance between second. control grid ii and connectedelectrodes l4 and I5 is negative. When, as in the illustrated circuit,the

. current flowing to connected electrodes II and II stream in dischargedevice I regeneratively by voltage on electrode ii. That is, in Fig. 1,it 1 appears as though a negative resistance etlect,

which causes regeneration for signals of audio frequencies, occursbetween connected electrodes I4 and i 5, second control grid II and thecathode I, considered as a group, since coupling condenser 22 is of lowreactance for signals of audio frequency.

In the particular amplifier circuit shown in Fig. 1, when properlyadjusted, the increase in triode gain in discharge device I due toregeneration is about 70%. It is possible, however. to increas theregenerationto such an extent that a gain of about 2000 is obtained inthe triode section alone. This condition is not satisfactorily stable.

In general, any means that causes a change in transconductance betweenthe control electrode II and anode I 3 maybe used to control negativetransconductance between control grid H and connected electrodes 14 andI5 so that o the amount of regeneration in discharge device sistance l8.When anode resistance l8 is'de-I creased below a critical resistance,the transconductance between the control grid i I and anode l3 isincreased so that device l breaks into oscillation, due to thecorresponding increase in negative transconductance between control gridH and connected grids l4 and i5. The signal voltage developed acrossresistance i8 is, of course, equal to the signal current flowingtherethrough, multiplied by the ohmic magnitude of resistance l8. Bymaking resistance l8 high in magnitude the transconductance of thedevice is lowered but the voltage developed across resistance tends toremain constant.

Characteristics and operating features of device i are further set forthin the co-pending application of John G. Prentiss, filed October 4.1943, Serial Number 504,958, and assigned to the same assignee as thepresent invention.

Tone control is produced by adjusting the position of the manuallyoperated movable element II which is rotatably mounted on insulatingsupport 42 by means of pin 43. (Fig. 5). Movable element ll has anintegrally formed projection ll which extends out from the housing orhearing aid case 45 for convenient manipulation by the finger of anoperator and. also has suitable indicia 46 for indicating to theoperator the operating position of movable element 4|. Movable elementll may be held in any one of the four positions by means of a resilientfinger 41, having one of its ends fastened to support member 42 by meansof rivet 48 and its free end arranged to enter-into one of a series offour equally spaced notches 49 on the'periphery of rotatably mountedelement 4|.

Rotationalmovement of element 4| is limited substantially to the angulardistance corresponding to the overall spacing of the four notches bymeans of the heads of rivets 50 and I|,re'- spectively engaging theopposite sides of the walls defining aperture 52 in support member 42.Rivets 50 and Si not only serve to limit rotational movement of element41 but also serve extends beyond the periphery of element H where suchbar has a width corresponding substantially to the angular distancethrough which element 4| may be rotated. Bar 38 comprises an inwardlydisposed body portion 39 and a projecting portion 40 which extendsoutwardly from the axis of element rotation so as to engage contactmembers 53 and/or 54 respectively, fixedly mounted On insulating support42 by means of rivets 55 and 58. A third contact member 51 fixed oninsulating support 42 by means of rivet 58 extends iurther inwardly thanthe other two contact members 53 and 54 so as to engage the body portion59 of bar 38 in all positions of element 4!. Contact members 53, 54 and51 are preferably equally spaced in that order at the periphery ofelement 4| with the overall distance from contact member 53 to contactmember 51 being of the order of the permissible angular movement ofelement 4i and with contact member 51 extending further inwardly in aradial direction than the other two contact members 53 and 54 such thatcontact member 51 continuously engages bar portion 3! and contactmembers 53 and 54 engage only bar portion 40 when it is in predeterminedpositions determined by the position of resilient finger 41' inaparticular notch 49. Bar projection 40 has a width substantially equalto the spacing between contact members 53 and 54 and so disposed as toshort circuit those members 53 and 54 in one predetermined position of.contact member 4| as shown in Fig. 2 and to engage respectively onlyone of the two contact members 53 and 54 in other predeterminedpositions of element 4| (Figs. 1 and 3). In the fourth predeterminedposition shown in Fig. 4 neither the contact member '53 nor the contactmember Si is engaged by bar portion 40.

As shown in Fig. 1, contact member 53 is connected through lead 31 toone terminal or condenser 35, contact member 54 is connected throughlead 36 to one terminal of resistance Hi, and contact member 58 isgrounded so as to continuously maintain the short circuit bar 3! atground potential for effecting the tone control previously described bymanipulating control element 4|. 7

Control element H is rotatably mounted not only to be easily operated bythe ringer oi the operator but also such as to be placed at the cornerof a hearing aid case 115 where it is readily accessible to the operatorand does not distract from the general appearance of the case. The case45 is preferably made with rounded corners so as to reduce noisesproduced by rubbing of the operators garments on the housing. As shownin Fig. '7, even though the tone control element 4! is mounted at acorner of the housing, the element H is so shaped and so moved andpositioned in its dirierent operating positions that the benefits of arounded corner are obtained together with easily accessibie and easilyoperated tone control.

In Fig. 7, the edge 59 of the hearingaid case 45 serves to obscurecertain indicia on control element 4! depending upon its particularposition. The arrow engraved on the hearing aid case points to theparticular predetermined position oi the tone control element 4! and theposition 3 shown in 1 corresponds to the position of element 38 shown inFig. 2.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may :be made without departing from thisinvention in its broader aspects, and, therefore, the aim in theappended claims is to cover all such changes and modifications as fallwithin the true spirit and scope of this invention.

I claim:

1. In combination, a support, a movable element rotatably mounted onsaid support, means for limiting the rotatable movement of said elementwith respect to said support to a relatively small arc 01' a circle,said element having spaced notches therein along a length thereofcorresponding to said are, three spaced insulated contact membersmounted on said support and arranged along said arc of said circle, ametallic short circuiting bar carried on the periphery of said rotatableelement and arranged to make contact with said contact members indiii'erent predetermined positions of said element, said her having awidth corresponding to the length of said arc, resilient means mountedon said support and :biased into said notches for maintaining saidmovable element in any one of a plurality oi predetermined positionswherein said bar is positioned differently relative to said contactmembers, said bar having an inwardly disposed body portion and aprojecting portion, said projecting portion extending outwardly from theaxis of rotation of said element and having a width at least as great asthe distance between two adjacent ones of said contact members, theother one of said contact members extending radially inwardly a distancesufficient to engage said body portion in all positions of said element,said notches having such spacing that in one position of said elementsaid projecting portion engages only one of said two adjacent contactmembers, in a second predetermined position :both of said adjacentcontact members are engaged by said projecting portion and in a thirdpredetermined position only the other one of said adjacent contactmembers is engaged by said projecting portion.

2. In combination, a support, a movable element rotatably mounted onsaid support, said support having an aperture, said element havingspaced notches therein along a length thereof, three spaced insulatedcontact members mounted on said support and arranged near the peripheryof said element, a metallic short circuiting bar, means for fasteningsaid bar on the periphcry of said rotatable element whereby said barmakes contact with said contact members in dii- Ierent predeterminedpositions of said element, said fastening means having a head thereonarranged to engage the walls defining said aperture of said supportwhereby the rotatable movement of said element with respect to saidsupport is limited to a relatively small arc of a circle, said barhaving a width corresponding to the length of said arc, resilient meansmounted on said sup port and [biased into said notches for maintainingsaid movable element in any one of a plurality of predeterminedpositions wherein said bar is positioned relative to said members, saidbar having an inwardly disposed bodyportion and a projecting portion,said projecting portion extending outwardly from the axis oi rotation ofsaid element and having a width at least as great as the distancebetween two adjacent ones of said contact members, the other one of saidcontact members extending radially inwardly a distance sulficient toengage said body portion in all positions of said element, said notcheshaving such spacing that in one position of said element said proiectingportion engages only one of said two adjacent contact members, in a'second predetermined position both of said adJacent contact members areengaged by said projecting portion and in a third predetermined positiononly the other one oi said adjacent contact members is engaged by saidprojecting portion.

3. In combination, an electron discharge, device having a first andsecond electrode, separate means including a control grid for providingin said device an electron discharge stream modulated in intensity atfrequencies within a predetermined band of frequencies, said firstelectrode being so arranged with respect to said stream as to develop analternating potential in response to the intensity modulation of saidstream, said second electrode being arranged to aflect said electronstream in response to potentials ims pressed on said second electrodeafter said stream aflects said first electrode, means forimpressingsubstantially all of said alternating potential developed by said streamon said first electrode upon said second electrode, whereby potentialsimpressed on said second electrode after being developed on said firstelectrode increase the efl'ect on said first electrode of intensitymodulation of the electron stream beyond the effect which would existwithout the impression of such alternating potential upon said secondelectrode, an electroacoustical transducer connected to said controlgrid, a first frequency selective impedance element for transferringselectively certain electrode, means for impressing substantially all ofsaid alternating potential developed by said stream on said firstelectrode upon said second electrode, whereby potentials impressed onsuch '.second electrode after being developed on said frequencycomponents of signals from-the transducer to said control grid, a secondimpedance element for altering the degree to which signals of diflerentfrequencies are amplified in said discharge device, and means foroperatively connecting said first and second mentioned imquencies, meansfor impressing a positive average potential on said first electrodewhereby said I stream develops an alternating potential on said firstelectrode in response to the intensity modulation of said stream, saidsecond electrode being arranged to afiect said electron stream inresponse to potential impressed on said second electrode after saidstream afiects said first electrode, means for impressing substantiallyall of said alternating potential developed by said stream on said firstelectrode'uponsaid second electrode, whereby potentials impressed onsaid second electrode after being developed on said first electrodeincrease the efl'ect on said first electrode of intensity modulation ofthe electron stream beyond the eflect which would exist without theimpression of such alternating potential upon said second electrode, animpedance element discharge stream modulated in intensity at irequencieswithin a predetermined band of frequencies, means for impressing anaverage positive potential on said first electrode whereby firstelectrode increase the efiect on said first electrode of intensitymodulation of the electron stream beyond the effect which-would existwithout the impression of such alternating potential upon said secondelectrode, an impedance element for altering the degree to which signalsof diflerent frequencies are amplified in said discharge device, andmeans for selectively connecting said impedance element to said firstelectrode.

6. In combination, an electron discharge device having a cathode, acontrol electrode and two anode electrodes, means for producing aregenerative efiect on one of said anode electrodes to increase theintensity of signals impressed on said control electrode and appearingon the other of said anode electrodes, means for impressing a signalhaving frequencies within a redetermined band of frequencies on saidcontrol electrode, an

impedance, and means for selectively connecting said impedance betweensaid cathode and one of said electrodes to alter the relative amount ofamplification of signals in said discharge device over said band oifrequencies.

7. In combination, an electron discharge device having a cathode, acontrol electrode and two anodes, means for producing a regenerativeeil'ect on one of said anodes 'to increase the intensity of signalsimpressed on said control electrode and appearing on the other of saidanodes, means for impressing a signal having frequencies within apredetermined band ofirequencies on said control electrode, two unlikeimpedances, and means for selectively connecting one of said impedancesbetween said cathode and said control electrode, the other of saidimpedances between said cathode and one of said anodes, both of saidimpedances between said cathode and respectively said control electrodeand said one anode simultaneously, and for disconnecting both of saidimpedances whereby the relative amount of amplification of signals insaid discharge device of said band frequencies is correspondinglyaltered.

8. In combination, a support, a movable element rotatably mounted onsaid support-means I limiting the rotatable movement of said elementwith respect to said support to a relatively small arc of a circle, saidelement having at least one positioning member thereon to hold saidelement in at least one predetermined adjusted position intermediate theextremities of said are along a length thereof corresponding to saidare, three spaced insulated contact :members mounted on said support andarranged along said arc of said circle, a metallic shortcircuiting barcarried on the-periphery oi said rotatable element and arranged to makecontact with said contact members in diilerent predetermined positionsof said element, said bar having a width corresponding to the length ofsaid are, at. least one cooperating positioning member mounted on saidsupport and cooperating with said first mentioned positioning member tohold said element in said one predetermined adjusted position such thatsaid bar may be positioned diflerently relative to said contact members,said bar having an inwardly disposed body portion and a projectinportion, said projecting portion extending outwardly from the axis ofrotation of said element and having a width at least as great as thedistance between two adjacent ones of said contact members, the otherone or said contact members extending radially inwardly a distancesufllcient to engage said body portion in all adjusted positions of saidelement, said cooperating positioning members being spaced such that inone position of said element said projecting portion engages only one ofsaid adjacent contact members, in a second predetermined position bothof said adjacent contact members are engaged by said projecting portion,and in a third predetermined position only the other one of saidadjacent contact members is engaged by said projecting portion.

9. in combination, a support, a movable element rotatably mounted onsaid support, means limiting the rotatable movement or said element withrespect to said support to a relatively small arc of a circle, saidelement having at least one positioning member thereon to hold saidelement in at least one predetermined adjusted position intermediate theextremities of said arc, three spaced insulated contact members arrangedalong said are of said circle, a metallic shortcircuiting bar arrangedto make contact with said contact members in dlflerent predeterminedpositions of said element, movement of said movable element producingrelative movement between said three insulated contact members and saidshortcircuiting bar, said bar having a width corresponding to the lengthof said arc, at least one cooperating positioning member mounted on saidsupport and cooperating with said first mentioned positioning member tohold said element in said at least one predetermined adjusted position,and arranged such that said bar may be positioned diiIerently relativeto said contact members, said bar having an inwardly disposed bodyportion and a projecting portion, said projecting portion extendingoutwardly from the axis of rotation of said element and having a widthat least as great as the distance between two adjacent ones of saidcontact members, the other one of said contact members extendingradially inwardly a distance sufllcient to engage said body portion inall positions of said element, said cooperating positioning membersbeing arranged such that in one adjusted position of said element saidprojecting portion eng ges only one of said two adjacent contactmembers, in a second predetermined adjusted position both of saidadjacent contact members are engaged by said projecting portion, and ina third predetermined adjusted position only the other one of saidadjacent contact members is engaged by said projecting portion.

EDWARD B. PASSOW-

